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arXiv 2203.06302astro-ph.EP
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极近行星大气逃逸造成的质量损失

Mass loss by atmospheric escape from extremely close-in planets

Tommi Koskinen, Panayotis Lavvas, Chenliang Huang, Galen Bergsten, Rachel Fernandes, Mitchell Young

备注：
Accepted to Astrophysical Journal
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摘要：

我们探索大气逃逸从近距离的系外行星具有最高的质量损失率。首先，我们定位从恒星X射线和紫外驱动的逃逸到快速洛希瓣溢出的过渡，一旦大气中10-100 nbar的压力水平到达洛希瓣，就会发生这种过渡。当沿着可见表面压力水平的亚星半径与极地半径之比（与恒定罗氏势的表面对齐）为X/Z~ $\gtrsim$ ~1.2（类木行星）（Mp~ $\gtrsim$ ~100 M $_{\Earth}$）和X/Z~ $\gtrsim$ ~1.02（亚类木行星行星）（$M_p \approx$ ~10--100 M $_{\Earth}$）时，行星进入此状态。在类太阳恒星周围，这种状态适用于半径约为3- 14 R $_{\Earth}$的行星的轨道周期小于2天。我们的研究结果与已知凌日行星的性质一致，可以解释已知系外行星种群中的亚木星沙漠。其次，我们提出了详细的数值模拟大气逃逸的行星，如天王星或海王星轨道接近太阳类恒星，支持上述结果，并指出有趣的热木星和次类木行星之间的质的差异。我们发现，热海王星与太阳的金属丰度氢和氦信封有相对更多的扩展上层大气比典型的热海王星，具有较低的电离分数和较高丰度的逃逸分子。这与现有的温暖海王星的紫外线过境观测是一致的，它可能提供一种方法来使用未来的观测和模型来区分太阳金属含量较高的大气层。

英文摘要：

We explore atmospheric escape from close-in exoplanets with the highest mass loss rates. First, we locate the transition from stellar X-ray and UV-driven escape to rapid Roche lobe overflow, which occurs once the 10-100 nbar pressure level in the atmosphere reaches the Roche lobe. Planets enter this regime when the ratio of the substellar radius to the polar radius along the visible surface pressure level, that aligns with a surface of constant Roche potential, is X/Z~$\gtrsim$~1.2 for Jovian planets (Mp~$\gtrsim$~100 M$_{\Earth}$) and X/Z~$\gtrsim$~1.02 for sub-Jovian planets ($M_p \approx$~10--100 M$_{\Earth}$). Around a sun-like star, this regime applies to orbital periods of less than two days for planets with radii of about 3--14 R$_{\Earth}$. Our results agree with the properties of known transiting planets and can explain parts of the sub-Jovian desert in the population of known exoplanets. Second, we present detailed numerical simulations of atmospheric escape from a planet like Uranus or Neptune orbiting close to a sun-like star that support the results above and point to interesting qualitative differences between hot Jupiters and sub-Jovian planets. We find that hot Neptunes with solar metallicity hydrogen and helium envelopes have relatively more extended upper atmospheres than typical hot Jupiters, with a lower ionization fraction and higher abundances of escaping molecules. This is consistent with existing ultraviolet transit observations of warm Neptunes and it might provide a way to use future observations and models to distinguish solar metallicity atmospheres from higher metallicity atmospheres.

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